When a new switching system is to replace an existing switching system, the subscriber lines are typically connected in parallel to both switching systems while the old system continues to provide service. The connections between the subscriber lines and the two switching systems are verified to minimize the possibility of service interruption when the new system is cut into service. Such interruptions could occur if, for example, a wiring error is made during the connection of the lines to the new system or an erroneous entry is made in the translation database defining the relationship between directory numbers and equipment numbers in the new system.
In one known test method, the subscriber lines are connected in parallel to both systems at a main distributing frame, and a trunk, referred to herein as an office-to-office trunk, is connected between the two systems. The pre-cutover test of a given subscriber line proceeds as follows. The directory number associated with the given subscriber line in the translation database of the new system, is transmitted over the office-to-office trunk to the old system. The old system responds by establishing a metallic connection between the office-to-office trunk and the main distributing frame terminal defined by the translation database of the old system as being connected to the subscriber line defined by the received directory number. The new system generates a contact closure on the office-to-office trunk. Since a metallic path is present from the office-to-office trunk to the point where the subscriber lines are connected in parallel to the two systems, a successful test is indicated when the contact closure is detected at the main distributing frame terminal defined by the translation database of the new system as being connected to the given subscriber line.
The known test method is, however, not suitable for testing subscriber lines served via digital carrier facilities, such as the AT&T SLC.RTM. carrier systems, when such facilities interface directly with digital switching systems. Digital carrier facilities reduce the number of long copper pairs that are extended from a central office by multiplexing the conversations of a number of subscribers on each digital facility. Such use of digital facilities was initially made in conjunction with analog switching systems where the facilities are received in the central office by a central office terminal. The central office terminal is, in turn, connected to the analog system by conventional analog lines. Each of these analog lines corresponds to one of the subscriber lines served via the digital facilities. The digital facilities interface with the subscriber lines at a remote terminal located near the customers' premises. When the analog switching system is replaced by a digital switching system, the central office terminal is, in some cases, removed and the digital facilities are interfaced directly with the replacement digital system. Unlike the existing analog system, the replacement digital system does not have an individual system termination associated with each of the subscriber lines. As a result, a metallic continuity check such as that used in the above-described test method cannot be used as a pre-cutover verification of the subscriber lines.
In view of the foregoing, a recognized problem in the art is the inadequacy of known pre-cutover test methods for verifying switching system subscriber lines served via digital carrier facilities.